Amplitude limiting system



Dec. 22, 1942.

N. P. CASE AMPLITUDE LIMITING. SYSTEM Filed Feb. 25, 1941 INVENTOR ELSON P. CASE ATTORNEY OZuDOmmu 0534 1 Eur-.313:

mOhOuhmQ Patented Dec. 22, 1942 AMPLITUDE SYSTEM Nelson P. Case, Great Neck, N. Y., assignor to Hazeltine Corporatlon, a corporation on Delaware Application February 25, 1941, Serial No. 380,447

11 Claims. (Cl. 178-44) The present invention relates to, amplitudelimiting systems and, particularly, to amplitudelimiting systems of the type employing a vacuumtube repeater for limiting to a predetermined amplitude level in an output circuit of the repeater signals applied to an input circuit thereof. While the invention is of general application, it has particular utility for use as a limiting system in a frequency-modulation carrier-signal receiver and will be described in that connection.

Frequency-modulation carrier-signal receivers generally use an amplitude-limiting system to reduce the undesirable eiTects of noise disturbances of the type whichappear as transient amplitude variations of the received carrier signal as well as other spurious amplitude variations.

The amplitude-limiting systems of the prior art conventionally comprises. vacuum-tube repeater,

all'of the electrodes of which, with the exception of the control electrode, are energized with poten- I tials of predetermined substantially constant values. As thus energized, the vacuum-tube repeater limits to a predetermined amplitude level in its output circuit only those input signals having intensities in excess of a predetermined average value. It is desirable that the limiting system limit at a relatively high amplitude level where the received carrier signal has ample intensity in order that full advantage may be taken of the better detector characteristics available at such input levels. However, in the event that the received carrier signal is of a relatively weak intensity, or in case the average intensity decreases due to the phenomenon of fading, it is nevertheless desirable that the spurious amplitude variations of the carrier signal be removed, even though this necessitates limiting at lower amplitude levels. Otherwise, the noise disturbances are reproduced and maybe sufiiciently strong that the relatively weak carrier signal is not usable. The aforementioned prior art limiting systems, which are capable of limiting at only one amplitude level, are expressly so designed and energized as to limit only on carrier signals of relatively strong intensity and this characteristic constitutes a distinct limitation on the range of carrier-signal intensities which may be satisfactorily reproduced by a frequency-modulation carrier-signal receiver embodying the prior art limiting systems.

It is an object of the present invention, therefore, to provide anew and improved amplitudelimiting system and one which avoids one or more of the disadvantages and limitations of the prior art systems of this nature.

It is a further object of the invention to provide an amplitude-limiting system wherein the limiting level thereoi remains substantially constant for applied signals of intensities in excess of a predetermined average value but is lowered for signals of intensity less than this predetermined average value.

' In accordance with the invention, an amplitude-limiting system comprises an input circuit adapted to have applied thereto signals the average intensities of which may vary over a wide intensity range, which signals are subject to the undesired amplitude variations. The system includes an output circuit, amplitude-limiting means coupling the input and output circuits for limiting to predetermined amplitude levels in the output circuit input signals, and means responsive to the average intensities of the applied signals for varying the limiting level of the limiting means in accordance therewith.

In a preferred form of the invention, an amplitude-limiting system of the type described comprises amplitude limiting means of the aforesaid type which includes a vacuum-tube repeater having a variable cutoff level, the limiting level of the limiting means varying with the cutoff level of the vacuum-tube repeater. cludes means responsive to signals of intensities less than the aforesaid predetermined average value for lowering the cutofi level of the vacuumtube repeater, as by lowering the energization thereof, in accordance with the applied signal intensities to lower the limiting level of the limiting system.

For a better understanding 6f the present invention, togetherwith other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a circuit diagram, partly schematic, representing a complete frequency-modulation carrier-signal receiver embodying the amplitude-limiting system of the present invention: and Fig. 2 is a graph representing certain operating characteristics of the limiting system of Fig. 1. 1

Referring now particularly to Fig. 1 of the drawing, there is represented schematically a complete frequency-modulation carrier-signal receiver of a conventional design embodying the present invention in a preferred form.- In general, the carrier-signal receiver includes a radiofrequency amplifier in having an input circuit connected to an antenna system II, I! and hav- The system in-- aeoaeea ing an output circuit connected to an oscillator- :modulator is. Connected in cascade with the oscillatonmodulator $3, in the order named, are an intermediate-frequency amplifier id of one or more stages, an amplitude-limiting system it, more'fully described hereinafter, a frequency detector 36, an audio-frequency amplifier ll of one or more stages, and a sound reproducer i8.

It will be understood that the various units just described may, with the exception of the amplitude-limiting system 115, be of a conventional construction and operation, the details of which are well known in the art, rendering detailed description thereof unnecessary. Considering briefly the operation of the receiver as a whole. and neglecting for the moment the operation of the amplitude-limiting system it presently to be described, a desired frequency-modulated carrier signal isselected and amplified by the radiofrequency amplifier Id, converted to a frequencymodulated intermediate-frequency carrier signal in the oscillator-modulator l3, amplified in the intermediate-frequency amplifier M, limited by the amplitude-limiting system IE to an average amplitude dependent in value on the average intensity of the received signal, and detected by the frequency detector l6, thereby to derive the audio-frequency modulation components. The audio-frequency components are, in turn, amplified in the audio-frequency amplifier l1 and are reproduced by the sound reproducer I8 in aconventional manner.

Referring now more particularly to the portion of the system embodying the present invention, the amplitude-limiting system l includes an input circuit comprising an input transformer l9 and an .output circuit comprising an output transformer 22, the input and output transformers being tuned by the respective condensers 20, 2| ahd 23, 24 to the intermediate-frequency carrier signal. The limiting system I!) includes amplitude-limiting means coupling the input and output circuits for limiting to a predetermined amplitude level in the output circuit input signals of intensities in excess of a predetermined average value. This means comprises a vacuum-tube repeater 25 having a variable cutoff level, the limiting level of the limiting system I5 varying with the cutoff level of this vacuum tube. The limiting means also includes means for developing from the signals unidirectional control bias, the magnitude of which varies with the average intensity of the applied signals, comprising a grid condenser 26 and a grid leak 21 for repeater 25. The anode and screen grid of vacuum tube 25 are energized through a resistor 28 from a suitable source of space current indicated as +B.

The limiting system sponsive only to signals of intensities less'than the aforesaid predetermined average value for lowering the limiting level of the limiting means in accordance therewith. This means comprises an auxiliary vacuum tube 29 and a circuit for applying the control bias developed across the resistor 21 to its control electrode comprising a filter network including a series resistor 3| and a shunt condenser 32, this network effectively coupling the. control electrode of tube 29 to the control grid of vacuum tube 25. The anode of vacuum tube 29 is coupled directly to the screen grid of tube 25 and is coupled to the source of space current +B through the re tor 28, which resistor is included in an impedance means common to the space current paths of the vacuum tubes.

l5 also includes means re- Considering now the just described, the input circuit transformer I9 of the amplitude-limiting system 15 is adapted to have applied thereto from the intermediate-- frequency amplifier carrier signals the average intensities of which may vary over a wide intensity range and which may be subject to undesired amplitude variations. Assuming that a particular carrier signal applied to the limiting system l5 has a relatively large intensity, there is developed from the carrier signal by grid rectification in the vacuum-tube repeater a unidirectional control bias or potential which appears across the resistor 21, the magnitude of this potential varying with the average intensity of the applied carrier signal. The umdirectional control potential is applied through the filter network 3|, 32 to the control electrode of the auxiliary .vacuum tube 29; As long as the carrier signal has relatively strong intensity, as here assumed, the unidirectional control potential is sufficient to bias the auxiliary vacuum tube 29 substantially to cutoff, whereby the latter draws applied to the input thereof a little or no current from the source of space current +B. Consequently, maximum energizing potential is applied to the screen grid and anode of the vacuum-tube repeater 25 and the latter has its maximum cut-off level so that the carrier signal is repeated in the output circuit of the limiting system IE, but is limited to a predetermined amplitude level therein.

' Referring to Fig. 2, a relatively strong intensity input-carrier signal represented by curve A develops across the resistor 21 by grid rectification a negative bias e1. Curve B represents the operating characteristic of the vacuum tube 25 with maximum energizing potential applied to the screen grid and anode thereof, as when the auxiliary vacuum tube 29 is biased to cutofl, and it will be evident that the repeated carrier signal. represented by curve C, which appears in the output circuit of the limiting system I! is limited at the cutoff level e: of the vacuum tube 29., This limiting action is substantially uniform for all input signals of intensities in excess of a predetermined average value at which sumcient bias potential is developed across the resistor 21 to bias the repeater vacuum tube 25 to cutofl. The amplitude-limited carrier signal is applied to the frequency detector I6 for detection thereby in normal manner. 1

Now assume that a carrier signal having relatively weak average. intensity, as represented by curve D, is applied to the input circuit transformer I9, 20 of the amplitude-limiting system IS. The magnitude of the unidirectional control potential e3 developed across the resistor 21 by grid rectification of vacuum tube 25 is correspondingly smaller and, were it not for the action of the auxiliary control tube 29, the weak intensity carrier signal would be repeated without limiting, as indicated by the dotted-line curve E, in the output circuit of the limiting The undesired amplitude variations of the carrier signal consequently would be applied to, and would ailect the operation of, the frequency detector l6. However, the magnitude of the developed unidirectional now suflicient to bias the auxiliary vacuum tube 29 to cutofl and the latter draws current from the space current source +B, thereby to increase the voltage drop across the resistor 29' which, in turn, reduces the value of the screengrid and anode potentials of the vacuum-tube repeater 25. This reduction of energization of operation of thexcircuit M intermediate-frequency system II.

control potential ea is not the vacuum tube 25 modifies the operating characteristic of this tube, as to that represented by curve F, and the cutoflf level of the vacuum-tube repeater 25 is reduced to a value c; corresponding in degree to the reduction of energization thereof thus eil'ected. The relatively weak intensity carrier signalisthus repeated in the output circuit of the limiting system I with limited amplitude, as represented by curve G. The limiting or cutofi level of the limiting system l5 consequently is efiectively lowered. The amplitude-limiting means which includes the vacuumtube repeater 25 is, therefore, effective, to limit input signals to predetermined amplitude levels change of conductance over the operating range of grid biases in order that decreasing negative values of bias potential developed acrossthe resistor 21 shall increase the space current of the auxiliary vacuum tube 29 rapidly to procure a sensitive control.

It will thus be seen that the auxiliary vacuum tube 29 comprises means responsive to signals of intensities less than a predetermined average value for lowering the cut-off level, asby decreasing the energization, of the vacuum-tube repeater 25 in accordance therewith to lower the limiting level of the amplitude-limiting system Hi. This action is efiected by the response of the auxiliary vacuum tube 29 to values of the control bias developed across the resistor 21 corresponding to signals of intensities less than the aforesaid predetermined average values. Further, it will be evident that the amplitudelimiting level of the limiting means, comprising the vacuum-tube repeater 25, is controlled over a range of limiting levels less than the aforesaid predetermined level inversely in accordance with variations of conductance of the auxiliary vacuum tube 29.

It will also be seen that the vacuum tube 29 constitutes a bias-controlled vacuum tube which is coupled to the limiting means including the vacuum-tube repeater 25 for controlling the limiting level thereof in accordance withthe control bias applied to the vacuum tube 28. Tube 29' is, therefore, a variable-conductance vacuum tube which is coupled to the limiting means for controlling the amplitude-limiting level thereof. It is also apparent that the filter network 3|, 32 is included in ameans which is responsive to the average intensities of the applied signals for varying the conductance of the vacuum tube 29 4.

in accordance therewith to vary the limiting level of the limiting means. Likewise, it is obvious that the vacuumtube 29 comprises a means which is responsive to the average intensities of the applied signals and the control bias developed therefrom for varying the cutoif level of the vacuum tube repeater, and for modifying or decreasing the energization of the vacuum-tube repeater 25 in accordance therewith for varying or lowering the limiting level of the limiting,

means. V

It will now be evident from the above description of the invention that an amplitudelimiting system embodying'the invention has the subject to undesired amplitude variations, an

output circuit, amplitude-limiting means including a vacuum-tube repeater having a variable advantages that a carrier signal applied to the limiting system is limited at a relatively ,high level when the intensity of the applied carrier signal is adequate to ensure good detection,

thereby to take advantage .of the better detecting characteristic of the frequency detector of the receiver, but thatthe limiting occurs at a lower.level when the desired carrier signal is of relatively weak intensity, whereby the reproduction of the carrier signalis accomplished with minimum eiiect from the undesired amplitude variations of the carrier signal.

While there has been described what is at present considered to be the preferred embodi ment of this invention, it will be obvious to those and modifications as fall within the true spirit and scope oi. the invention.

What is claimed is: 1. An amplitude-limiting system comprising,

an input circuit adapted to have applied thereto signals the average intensities of which may vary over "a wide intensity range, said signals being subject to undesired amplitude variations, an output circuit, amplitude-limiting means coupling said input and output circuits for limiting to a predetermined amplitude level in said output circuit input signals of intensities in excess of a predetermined average value, and means responsive only to signal intensities less than said predetermined average value for varying the limiting level of said limiting means in accordance therewith.

-2. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals the average intensities of which may vary over a wide intensity range, said signals being cutoff level for couplingsaid input and output circuits to limit to predetermined amplitude levels in said output circuit input signals, and means responsive to the average intensities of applied signals for varying the cutoff level of said vacuum-tube repeater'in accordance therewith to vary the limiting level of said limiting means.

3. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals the average intensities of which may vary over a wide intensity range, said signals being subject to undesired amplitude variations, an

output circuit, amplitude-limiting means including a vacuum-tube repeater coupling said input and output circuits for limiting to predetermined amplitude levels in said output circuit input signals, and means responsive to the average intensities of applied signals for modifying the energization of said vacuum-tube repeater in accordance therewith to vary the limiting level of said limiting means.

4. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals the average intensities of which may vary over a wide intensity range, said signals being subject 'to undesired amplitude variations, an

output circuit, amplitude-limiting means including a vacuum-tube repeater coupling said input and output circuits for limiting to a predeter mined amplitude level in said output circuit input signals of intensities-in excess of a predetermined average value, and means responsive only to decreasing signal intensities less than said predetermined average value for decreasing the energization of said vacuum-tube repeater in accordance therewith to lower the limiting level of said limiting means.

5. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals the average intensities of which may vary over a. wide intensity range, said signals being subjectto undesired amplitude variations, an output circuit, amplitude-limiting means coupling said input and output circuits for limiting to predetermined amplitude levels in said output circuit input signals, a variable-conductance vacuum tube coupled to said limiting means to control the amplitude-limiting level thereof, and means responsive to the average intensities of applied signals for varying the conductance of said vacuum tube in accordance therewith to vary the limiting level of said limiting means.

6. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals the average intensities of which may vary over a wide intensity range, said signals being subject to undesired amplitude variations, an output circuit, amplitude-limiting means coupling said input and output circuits for limiting to predetermined amplitude levels in said output circuit input signals, said limiting means including means for developing a unidirectional control bias themagnitude of which varies with theaverage intensity of said applied signals, and means responsive to said control bias for varying the limiting level of said limiting means in accordance therewith.

7. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto with the'conductance of said auxiliary vacuum tube, and means responsive to signal intensities for varying the conductance of said auxiliary vacuum tube in accordance therewith to vary the cutofi level of said vacuum-tube repeater, thereby to vary the limiting level of said limiting means. i

9. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals theaverage intensities of which may vary magnitude of which varies with the average insignals the average intensities of which may vary over a wide intensity range, said signals being subject to undesired amplitude variations, an output circuit, amplitude-limiting means coupling said input and output circuits for limiting to predetermined amplitude levels in said output circuit input signals, said limiting means including means'for developing a unidirectional control bias the magnitude of which varies with the average intensity of said applied signals, a bias-controlled vacuum tube coupled to said limiting means to control the limiting level thereof in accordance with the control bias applied to said vacuum tube, and means for applying said control bias to said vacuum tube. 8. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals the average intensities of which may vary over a wide intensity range, said signals being subject to undesired amplitude variations, an output circuit, amplitude-limiting means including a vacuum-tube repeater having a variable cutofl level for coupling said input and output circuits to limit to predetermined amplitude levels in said output circuit input signals, the limiting level of said limiting means varying with the cutofl! level of said vacuum-tube repeater, means including an auxiliary vacuum tube having a variable conductance for controlling the cutoff level of said vacuum-tube repeater inversely in accordance tensity of said applied signals, an auxiliary vac- ,uum tube coupled to said limiting means to control the amplitude-limiting level thereof in accordance with variations of conductance of said auxiliary vacuum tube, and means for applying said unidirectional control bias to said auxiliary vacuum tube to vary the conductance thereof for values of said control bias corresponding to signals of intensities less than said predetermined average value, thereby to vary the limiting level of said limiting means for signals having said latter intensities.

10. An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals the average intensities of which may vary over a wide intensity range, said signals being subject to undesired amplitude variations, an output circuit, amplitude-limiting means including a vacuum-tube repeater coupling said input and output circuits for limiting to predetermined amplitude levels in said output circuit input signals, the limiting level of said limiting means varyng with the energization of said vacuum-tube repeater, an auxiliary vacuum tube, impedance means common to the space current paths of said vacuum tubes, and means responsive to the intensitles of said applied signals for controlling the space current of said auxiliary vacuum tube to control the energization of said vacuum-tube repeater, whereby the limiting level of said limiting means is controlled in accordance with the intensities of said applied signals:

lL-An amplitude-limiting system comprising, an input circuit adapted to have applied thereto signals the average intensities of which may vary over a wide intensity range, said signals being subject to undesired amplitude variations, an output circuit, amplitude-limiting means coupling said input and output circuits for limiting to predetermined amplitude levels in said output circuit input signals, and means responsive to the average intensities of said applied signals for varying the limiting level of said limiting means in accordance therewith. v

' NELSON P. CASE. 

